Session P23: Focus Session: Multifunctional Oxides: BiFeO3 and Thin Films

Strain tunability and domain structures of epitaxial (001) BiFeO$_{3}$ thin films

It was recently discovered that the spontaneous polarization ($P_{s})$ values determined in epitaxial BiFeO$_{3}$ thin films, $\sim $100 $\mu $C/cm$^{2}$, are over an order of magnitude higher than those previously measured in bulk samples. This raises a fundamental question: can the remanent polarization and other properties of BiFeO$_{3}$ be tuned by strain? We studied the strain dependence of remanent polarization and domain structures of BiFeO$_{3}$ through direct measurements on the \textit{same} epitaxial (001)$_{p}$ BiFeO$_{3}$ thin-film capacitors before and after releasing them from an underlying Si substrate. Our measurements reveal that: (1) the large $P_{s}$ of BiFeO$_{3}$ is indeed intrinsic; (2) the out-of-plane polarization ($P_{3})$ of (001)$_{p}$-oriented BiFeO$_{3}$ thin films has a strong strain dependence. These findings can be exploited in studying symmetry-dependent magnetoelectric coupling of BiFeO$_{3}$, where strain and/or symmetry play a role in the coupling because the direction of magnetic spin ordering is not parallel to that of ferroelectric polarization switching.   

Local Electrical Conductivity of Multiferroic Domain Walls

There is an intense interest in magnetoelectric coupling between electric and magnetic due to its potential to the revolutionary of device architectures. Single-phase multiferroics - materials that show spontaneous magnetization and polarization simultaneously at ambient conditions – remain elusive as most systems (such as the manganites) exhibit multiferroicity only at low temperatures. Alternatively, multiferroics can be synthesized as a composite system, e.g. as a product property of a composite phase consisting of a magnetostrictive and a piezoelectric material. One multiferroic material, however, has played a key role in rejuvenating the field after a report of large ferroelectric polarization combined with interesting magnetic properties - BiFeO$_{3}$. Here we provide evidence of a unique property of single domain walls in multiferroic BiFeO$_{3}$. Unlike other multiferroic materials, e.g. PbTiO$_{3}$, on which the electronic properties of the domain walls are not significantly different from the domain area, we observe a finite electric conductivity at room temperature along such a wall using conductive atomic force microscopy. This intrinsic property of the domain wall is attributed to a changed crystallographic structure as revealed by high resolution transmission electron microscopy. Additionally, optical absorption measurements confirm a change in bandstructure at domain walls in BiFeO$_{3}$.   

The multiferroic properties of Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ compound

Dense Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ ceramics of $R3c$ crystal structure were synthesized by solid state reaction under high pressure. TEM observations reveal superstructure characteristics in Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$. Magnetization as well as dielectric properties of Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ ceramics were characterized over a broad temperature.   

Optical coupling to spin waves in the cycloidal multiferroic BiFeO$_3$

The magnon and optical phonon spectrum of an incommensurate multiferroic such as BiFeO$_3$ is considered in the framework of a phenomenological Landau theory. The resulting spin wave spectrum is quite distinct from commensurate substances due to soft mode anisotropy and magnon zone folding. The former allows electrical control of spin wave propagation via reorientation of the spontaneous ferroelectric moment. The latter gives rise to multiple magneto-dielectric resonances due to the coupling of optical phonons at zero wavevector to magnons at integer multiples of the cycloid wavevector. These results show that the optical response of a multiferroic reveals much more about its magnetic excitations than previously anticipated on the basis of simpler models.   

Electric Field Controlled Magnetism in BiFeO$_{3}$/Ferromagnet Films

Electric field control of magnetism is a hot technological topic at the moment due to its potential to revolutionize today's devices. Magnetoelectric materials, those having both electric and magnetic order and the potential for coupling between the two, are a promising avenue to approach electric control. BiFeO$_{3}$, both a ferroelectric and an antiferromagnet, is the only single phase room temperature magnetoelectric that is currently known. In addition to other possibilities, its multiferroic nature has potential in the very active field of exchange bias, where an antiferromagnetic thin film pins the magnetic direction of an adjoining ferromagnetic layer. Since this antiferromagnet is electrically tunable, this coupling could allow electric-field control of the ferromagnetic magnetization. Direction determination of antiferromagnetic domains in BFO has recently been shown using linear and circular dichroism studies. Recently, this technique has been extended to look at the magnetic domains of a ferromagnetic grown on top of BFO. The clear magnetic changes induced by application of electric fields reveal the possibility of electric control.   

Investigation of domain walls in multiferroic BiFeO$_{3}$

We present a thorough study of domain walls in multiferroic BiFeO$_{3}$ thin films using scanning probe methods, transmission electron microscopy, transport measurements coupled with theoretical studies. In rhombohedral BiFeO$_{3}$, three different domain wall orientations exist, namely 180, 109, and 71 degrees. We find that the domain configurations in thin films are strongly dependent on the processing conditions. Here we investigate electrical and structural properties of all three varieties. Atomic resolution TEM studies were used to reveal the structure across the domain walls, with a specific focus on 109$^{\circ}$. We observe that the changed crystallographic structure in domain walls gives rise to a change in local properties. From the investigation of individual domain walls we also infer their relation to changed macroscopic properties in thin films. This work is supported by the US DOE, ONR MURI, NSF Chemical Bonding Center program, and the Alexander von Humboldt Foundation.   

Optical properties of BiFeO$_3$ thin films and single crystal

BiFeO$_3$ has been studied using optical and magneto-optical spectroscopy and the results are compared with first-principle calculations. The optical gap in thin films (2.7 eV) is much larger than that in the single crystal (1.3 eV), the evidence that this system has a novel transition between strongly correlated and band insulator behavior. Both magnetic field and temperature suppress the excitation between the strongly hybridized valence levels and Fe d levels near 1.5 eV. The temperature and magnetic field effects appear to scale energetically suggesting spin-charge coupling. The high energy magneto-dielectric contrast shows a jump at 18 T corresponding to the transition from toroidal to homogeneous antiferromagnetic phase.   

Magnetization manipulation in multiferroic devices.

Controlling magnetization by purely electrical means is a a central topic in spintronics. A very recent route towards this goal is to exploit the coupling between multiple ferroic orders which coexist in multiferroic materials. BiFeO3 (BFO) displays antiferromagnetic and ferroelectric orderings at room temperature and can thus be used as an electrically controllable pinning layer for a ferromagnetic electrode. Furthermore BFO remains ferroelectric down to 2nm and can therefore be integrated as a tunnel barrier in MTJ's. We will describe these two architecture schemes and report on our progresses towards the control of magnetization via the multiferroic layer in those structures.   

Electronic and magnetic structures of double perovskite multifunctional La$_{2}$NiMnO$_{6}$ thin films

Electronic and magnetic structures of the ordered double perovskite La$_{2}$NiMnO$_{6}$ (LNMO) thin films grown by pulsed laser deposition have been studied by x-ray absorption spectroscopy (XAS) and magnetic circular dichroism spectroscopy (XMCD). Based upon the results of XMCD, we find that the primary ion valence states to be Mn$^{4+}$/Ni$^{2+}$, and the ferromagnetism resulting from Mn$^{4+}$ -O - Ni$^{2+}$ superexchange interaction. Additionally, we show that the LNMO samples contain some Mn$^{3+}$ and Ni$^{3+}$ Jahn-Teller ions caused by oxygen or cation related defects. The orbital and spin magnetic moments of the Mn 3$d $and Ni 3$d $also have been deduced from the magneto-optical sum rules and compared with magnetization measurements.   

Influence of oxygen concentration on the magnetic properties of multifunctional La$_{2}$CoMnO$_{6}$ thin films

The dependence of the magnetic properties on oxygen concentration in epitaxial La$_{2}$CoMnO$_{6 }$(LCMO) thin films has been investigated grown by pulsed laser deposition (PLD). Using x-ray magnetic circular dichroism spectroscopy (XMCD) at Mn-$L_{2,3}$ and Co-$L_{2,3}$ edges, we have determined that the primary ion valence state is Mn$^{4+}$/Co$^{2+}$. Additionally, we see evidence of some low-spin Co$^{3+}$ ions, corresponding to the existence of a second ferromagnetic (FM) phases in our samples. The existence of oxygen vacancies induces the local vibronic Mn$^{3+}$-O-Co$^{3+}$ superexchange interactions in direct competition with the static FM Mn$^{4+}$-O-Co$^{2+}$ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions.